This invention relates to digital information signals, and in particular to systems in which timing information is to be extracted from a digital signal.
A digital signal normally represents digital information by the value of the signal at set times. In interpreting the signal those times must be known. In many systems they are defined by a separate clock signal which is used both by the apparatus that creates the signal and by the apparatus that responds to it. But in some instances, for example when the digital signal has been subjected to an unknown or variable delay, the relationship between the signal and the original timing information used in generating it is lost. Such delays occur when a digital signal is recorded magnetically and then read back, and in communication over long distances, for example by satellite. It is then desirable to be able to extract the timing information to be used in interpreting the signal from the signal itself. Signals which allow that to be done are often described as "self-clocking".
The channel introducing the delay is often bandwidth limited. Bandwidth limitation introduces distortion and makes recovery of the timing information more difficult. Thus a digital signal which originally consisted of sharply localised elementary signals is changed into one in which the elementary signals are depressed and spread out. If the width of each elementary signal spreads far enough, elementary signals at the minimum separation will overlap and interfere with one another.
Magnetic recording is an illustration of the way that makes recovery of timing information more difficult. The original digital signal consists of a write current in which the elementary signal is a reversal in the direction of the current. There is a minimum separation between reversals, but one reversal is not necessarily followed by another at the minimum separation. Each reversal is recorded on the magnetic medium as a transition between two regions magnetised to saturation in opposite directions and the transition, when read back, produces a bell-shaped read voltage. Now one method of extracting timing information is to detect the peaks of the read signal, on the assumption that each peak indicates the peak of one of these elementary bell-shaped pulses. But if the pulses interfere each peak will be shifted, by a variable amount which depends on whether or not the pulse has neighbours on either side at the minimum separation. Various complicated and unsatisfactory methods have been proposed to compensate for this peak shifting.